The present invention pertains to fluoride ion sensitive material suitable for use in the manufacture of active components for fluoride ion-sensitive electrodes for electro-analytic monitoring purposes. The present invention further provides a method for manufacturing said material of the invention which allows the use of polycrystalline starting materials and does not require highly reactive atmospheres used by some of the known methods of making fluoride ion sensitive materials.
The conventional fluoride ion-sensitive electrode (FISE) developed by Frant and Ross in 1966 utilizes a single crystal of LaF.sub.3 doped with EuF.sub.2 as its sensing element. The crystal has low solubility and low resistance, both of which are desirable for a low detection limit and a fast response. That electrode exhibits Nernstian behaviour from saturated to 10.sup.-5 M solutions of F.sup.-, and a detection limit of below 10.sup.-6 M F.sup.-. The only significant interferent is the OH.sup.- ion and this can be avoided by operating the system in acid solution.
The FISE is second only to the glass pH electrode, in terms of its selectivity, and few other ion sensitive electrodes can match its speed of response, range of operation and stability; all features making it very suitable for use as the sensing element in a biosensor. Particular applications include detection of organofluoride compounds by use in conjunction with such release agents as peroxidases. In this technique peroxidase is use to cause fluoride ion release from said organofluoride which may then be measured as related to said fluoride level. Thus the FISE may be coupled to any reaction scheme that releases F.sup.- ions which can be detected rapidly with no significant interference.
As a disposable biosensor the FISE is limited by the high cost of the single crystal device and thus an alternative more easily produced low cost device would be advantageously provided with such use in mind. However the excellent properties of the current FISE have resulted in few reports of alternative methods of production.
MacDonald and Toth, Anal. Chem. Acta. 1968, 41, 99, abandoned their work on electrodes produced from polycrystalline, low-solubility fluoride compounds supported in an inert matrix of materials such as polyvinylchloride or silicone rubber soon after the Frant and Ross electrode was described due to the latter devices superiority.
The patent relating to the single crystal material, U.S. Pat. No. 3,431,182, showed that several methods for the manufacture of FISEs had been investigated using insoluble monocrystalline and polycrystalline fluoride salts; monocrystalline LaF.sub.3, PrF.sub.3, CeF.sub.3, NdF.sub.3 and polycrystalline PbF.sub.2 and BiF.sub.3 were tested. The monocrystalline devices were shown to have the same disadvantage of expense as the conventional device, while the polycrystalline electrodes exhibited poor responses and were more susceptible to interference.
Improved performance electrodes have been made from polycrystalline starting materials by Hirata et al, as reported in Chem. Lett., 1974, 1451, whereby a mixture of LaF.sub.3, CaF.sub.2, and EuF.sub.3 was heated under pressure in an atmosphere of hydrogen fluoride gas to a temperature of 1200.degree. C. These workers reported that their material had slightly improved electrode characteristics compared with the single crystal device but the process used to produce it was complex.
To date possibly the least expensive method for providing fluoride sensitive electrode material having satisfactory properties is that provided by East German Patent DD 227,800 wherein a silicon disc was vacuum coated with LaF.sub.3 and bonding a wire to the rear of the disc using silver-loaded epoxy resin. The disc was then sealed into a conventional electrode body using epoxy resin. As can be seen, no satisfactory electrode material forming process yet exists which is capable of providing satisfactory electrodes from polycrystalline starting materials which does not use hazardous atmospheres or complex methods.